At present, touch detection technology for capacitive touch sensors mainly includes two kinds: the mutual capacitance detection and the self capacitance detection. Because the mutual capacitance detection has the feature of multi-touch, the mutual capacitance detection becomes the mainstream of the capacitive touch detection technology. However the mutual capacitance detection has some defects, such as the floating effect. The floating effect means the touch terminal is on the surface of a high insulation object; non-floating means a person contacts the touch terminal or a system ground with large area exists in the touch terminal. The floating effect means the phenomenon that when a touch is executed through a larger finger such as the thumb, while the touch area is smaller than that caused by other fingers. With the higher and higher user experience requirement, the floating effect has become the most important defect in the performance of the product using the mutual capacitance detection.
The principal of the floating effect of the mutual capacitance is shown in FIGS. 1 and 2, in the figures, Rh and Ch are respectively the impedance and capacitive reactance between the body and ground, two fingers touch different nodes. FIG. 1 is the principle diagram of the mutual capacitance in non-floating state. In the non-floating state, because Ch is large, a signal has a larger coupling with the ground through the Rh and Ch, thereby reducing the node capacitance by cutting the magnetic sensing line off, and detecting a corresponding variation. FIG. 2 is the principle diagram of the mutual capacitance in floating state. In the floating state, Ch is very small, the signal can hardly pass Ch, a drive signal for D1 is coupled to the sensing line S1 through the capacitance at node A, and the signal in S1 is finally coupled to the sensing line S2 through the capacitance at node C, finger and node B, such that the capacitance detected from S2 is larger compared to the capacitance in the non-floating state. When the press is executed by the thumb or an object with large area, the unexpected phenomenon that the capacitance at the center of the position of the press increases while the peripheral capacitance decreases will appear due to the floating effect.
The problem caused by the floating effect is mainly that using the thumb to press will cause point-split and judgment for the large area is difficult. The point-split means in the floating state, when the capacitive touch screen is touched by the object with large area (for example the thumb), the touch is considered as the multi-touch because of the signal cancellation. Referring to FIG. 3, in the non-floating state, when the touch executed by single finger causes large press area, it judges that one finger (the box drawn by bold lines in FIG. 3) exists though the mutual capacitance detection, and this judgment is correct. But in the floating state, referring to FIG. 4, when the touch executed by single finger causes large press area, it judges incorrectly that two fingers (two boxes drawn by bold lines in FIG. 4) exist. The method to solve that problem comprises using a metal housing, increasing the ground area of the device, reducing the coupling capacitances between the driving means and the finger, and the sensing means and the finger. The metal housing has the limitation that it must be contacted by the fingers, increasing the ground area has the product limitation, and reducing the coupling capacitances between the driving means and the finger, and the sensing means and the finger will affect the signal-to-noise ratio. Hence, all the existing methods can't resolve that problem fundamentally.